How Often Should You Exercise?

By Alex Hutchinson

Runner's World

Is it better to work out four times a week than six times a week? That's the finding of a new study in Medicine & Science in Sports & Exercise, which Gretchen Reynolds reported on in the New York Times a few days ago. The study reflects recent research trends in fitness, sparking debates on whether Too Much Exercise Could Kill You.

For this particular study, 72 volunteers—all sedentary women between the ages of 60 and 74—were divided into three groups for a 16-week training program:

one day of aerobic exercise and one day of resistance exercise per week;

two days of aerobic exercise and two days of resistance exercise per week;

three days of aerobic exercise and three days of resistance exercise per week.

The workouts progressed until they reached 40 minutes of aerobic exercise at 80% max heart rate, and two sets of 10 reps for 10 different resistance exercises at 80% of one-rep max.

What the researchers were really interested in discovering was how the exercise program affected the number of calories burned outside of exercise, which they measured with a sophisticated technique that involves tracking how molecules of "doubly labeled water" are processed through the body.

Their hypothesis was that two days of exercise per week wouldn't be enough to promote significant fitness gains, but that six days would be too much—the stress and time commitment would cause the hardest-training women to burn fewer calories during the rest of the day. Surprisingly, all the groups gained roughly the same amount of fitness; but as expected, the six-times-per-week group actually burned the fewest total number of calories each day. The four-times-per-week group burned the most calories.

So the conclusion reported in the study is that "twice per week training is more successful than more frequent or less frequent training." (In this case, they mean two aerobic workouts and two resistance workouts, for a total of four days.) And as Reynolds points out, those working out even less frequently than that apparently get almost the same benefits.

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